The present invention generally relates to radio transmitters capable of both digital and analog modulation for impressing information on the transmitted signal, and in particular to personal portable communications devices such as cellular phones using frequency modulation or quadrature modulation.
In U.S. Pat. No. 5,745,523, filed Oct. 27, 1992, the entirety of which is incorporated herein by reference, a dual mode radio apparatus having both a digital information transmission mode and an analog transmission mode is described. The modulation in either the digital or analog mode is applied by first computing In-phase (I) and Quadrature (Q) signals representative of the desired modulated signal vector and applying the I,Q signals through I and Q D/A converters to a quadrature modulator.
U.S. Pat. No. 5,020,076 to Cahill discloses a hybrid modulation apparatus that in a digital modulation mode applies I,Q modulation to a quadrature modulator, and in an analog modulation mode applies an analog frequency modulation waveform to a phase lock loop to generate a signal which is then passed straight through an I,Q modulator which is biased with constant I,Q signals.
It would be desirable to generate the analog modulation waveform needed in the Cahill method by using a digital signal processor employed for digital modulation to compute a sampled digital representation of the analog modulation waveform, and then converting the digital representation to the required analog modulation waveform using either the I D/A converter or the Q converter or both.
It would further be desirable for an analog modulation implementation to avoid the Cahill technique of passing the modulated signal through a permanently-biased I,Q modulator, since the Cahill method may sometimes result in a digital modulation frequency which is undesirable for analog modulation.
The present invention is directed toward a transmitter/receiver, such as a radiotelephone, which is capable of operating in two modes. In a first mode, the transmitter signal is modulated with digital information. Specifically, a digital signal processor computes sampled digital representations having a real or In-phase waveform (I) and an imaginary or Quadrature waveform (Q).
After digital-to-analog conversion in respective I and Q D/A converters, a quadrature modulator impresses the I,Q signals on an intermediate radio frequency. The intermediate frequency is subsequently upconverted to a desired transmission frequency by a local oscillator signal from the radiotelephone receiver.
In a second mode, the transmitter signal is modulated with an analog signal. Specifically, the digital signal processor forms the analog signal by computing a sampled digital representation of the analog modulation signal. The sampled digital representation is converted to an analog waveform using either or both of the I or the Q D/A converter, and applied to an analog modulator to produce an analog modulated radio signal that may be at a second intermediate frequency. This second intermediate frequency signal is subsequently converted to the desired transmission frequency with the aid of a local oscillator signal from the radiotelephone receiver.
In a preferred implementation, both the digital modulation and the analog modulation are constant envelope modulations that vary only the signal""s phase angle. A preferred method of transferring the desired angle modulation to the desired transmission frequency is to use a voltage-controlled oscillator to produce a signal at the desired transmission frequency and mix the signal with a local oscillator signal from the receiver to produce an intermediate frequency signal. The intermediate frequency signal is phase-compared with either a modulated or unmodulated reference signal to produce a feedback signal to control the oscillator to follow the desired angle modulation waveform. The feedback loop bandwidth is furthermore adapted according to the modulation mode selected either to follow digital modulation on the reference signal or not to follow analog modulation applied to the voltage controlled oscillator.